1. Field of the Invention
The present invention relates to a method for creating a color conversion table and an information processing apparatus, in particular, it relates to a method for creating a lookup table (hereinafter, referred to as a LUT simply) used in a color conversion process which generates data used in an image output apparatus such as a printer, and an image processing apparatus which executes a creating is process of the LUT.
2. Description of the Related Art
In image outputs by an inkjet printer or the like, generally a color conversion process is executed to convert image data of an input color signal from an image input device into image data of an output color signal used in the printer as an image output apparatus, to generate data for output. The input color signal is specified as a color of device color space (such as RGB color space) dependent on the image input device. On the other hand, the output signal is specified as a color of device color space (such as CMYK and RGB) dependent on the image output device. Therefore, the color conversion process is executed for converting the device color space of the input color signals into the color space of the output image signal. Conventionally, the color conversion from the device color space of an input device to the device color space of an output device has been realized by matrix operation. Currently, however, the color conversion often uses a three dimension LUT to acquire higher precise conversion.
The color conversion for color reproducibility (or reproducibility of a color gamut) is executed as one mode of the above-described color conversion. There are various types of color reproducibility such as color reproducibility with emphasis on tone, color reproducibility with emphasis on chroma, color reproducibility with emphasis on coincidence of colorimetry, and color reproducibility with emphasis on memory color. A prior art prepares a plurality of LUTs provided for a plurality of types of such color reproducibility, and selects an LUT according to the kind of image to be output to execute color conversion. Thus, color reproduction suitable for the image to be output can be realized. Also, in order to maintain this color reproducibility satisfactorily, the color conversion from the color space of an input device to the color space of an output device may be controlled by dividing the color conversion into two conversions, a pre-color conversion (color correction; gamut mapping) which executes the conversion of mapping of the color gamut and a post-color correction (color separation) which executes the color separation into data of printing agents such as ink.
Many kinds of color spaces are known as input color spaces generally used to specify image data, for example, sRGB and AdobeRGB for an RGB system and NTSC. To execute the color conversion from the device color space of the input device into the device color space of the output device correspondingly to a plurality of such input color space, at every combination of input and output devices an LUT specified for the color conversion between the combination must be provided. On the other hand, a color conversion system using an ICC profile which can execute color conversion regardless of the combination of input and output devices is also known. The ICC profile specifies the color conversion from input color space into device independent color space (for example, XYZ and Lab), correspondingly to the input device of the input color space; and likewise specifies the color conversion from the device independent color space into the device color space of the output device, correspondingly to the output device of the output color space. The color conversion which the profile specifies is concretely realized with LUTs just as described above. The color conversion using the ICC profile basically requires that each input and output device prepares just one profile specifying conversion relationship with device-independent color space. And, the color conversion of device color space between the input and output devices can be performed by combining the profiles of each input and output device. Additionally, the ICC profile holds an LUT having three properties, i.e., the color reproducibility with emphasis on tone, the color reproducibility with emphasis on chroma, and the color reproducibility with emphasis on coincidence of colorimetry, in one profile.
The LUT for the color conversion from the device color space of the input device into the device color space of the output device directly, and the LUT as the profile specifying color conversion relationship between the device independent color space and each device color space of the input and output devices are both created as follows.
First, target colors corresponding to grid points constituting the LUT to be created are determined. Next, device color of the output device (for example, post-RGB (RGB generated in the above-described pre-color conversion) or CYMK) which is most approximate to the target color is searched, and the device color is set as the color corresponding to the grid point, i.e., the grid point data. The target colors and the device colors based on the target colors are usually specified in a uniform color space (Lab or Luv).
More specifically, first, based on post-RGB (or CMYK) which is a color signal of the device color space of the printer, a predetermined number of color patches of these colors are printed out by the printer. Then, the color patches are measured to acquire the color signal of uniform-color color space corresponding to the color patches. Thus, the relationship between a post-RGB value (or a CMYK value) and a Lab value (or a Luv value or the like) of the uniform-color color space can be acquired. Additionally, when determining the relationship from the post-RGB value to the Lab value, although as the number of samples such as the color patches increases the precision becomes greater, it is unrealistic to print out all colors of the color space of the post-RGB as a sample. Thus, the color patches of colors of which the distance in the device color space of the printer is, for example, equidistant are printed out and measured to determine the relationship between the post-RGB values and the Lab values. Accordingly, a table from the printer-dependent color space to uniform color space (hereinafter, referred to as a color-space correspondence table) can be obtained. Additionally, for colors (post-RGB values) other than colors of post-RGB values which output the color patches, this color-space correspondence table is used to compute a known interpolation operation such as a tetrahedron interpolation to estimate Lab values, and thus similar relationships can be determined.
Next, a target color (a Lab value) corresponding to an object grid point constituting the LUT to be created and Lab values corresponding to all post-RGB values of the color-space correspondence table are compared to search the most approximate point which is a point at which the color difference is minimum. Thus, the post-RGB value of the most approximate point can be determined as a color corresponding to the object grid point, i.e., grid point data.
However, this method requires an enormous amount of time to search all post-RGB values. Japanese Patent No. 3371964 discloses a method to reduce the search time. In this document, first, a point at which a color difference with the target color is minimum is searched among post-RGB values corresponding to a measured color patch. Next, a point which can be approximated as a point at which the color difference with the target color is minimum is searched among post-RGB values in a neighboring region of the searched post-RGB value, and the post-RGB value is set as the color corresponding to the grid point. By reducing the search region, the search time is reduced.
However, even with the method in consideration of reducing the search time disclosed in Japanese Patent No. 3371964, the method still leaves room for reductions in search time, that is, the search time has not been reduced sufficiently. Although the method of Japanese Patent No. 3371964 searches a point at which the color difference with a target color is minimum among post-RGB values corresponding to the measured color patch in order to circumvent searching all post-RGB values, the method still executes a search, which requires a certain amount of time.
In addition, the neighboring region to be searched needs to contain midpoints between the grid point at which the color difference is minimum and adjacent grid points, and thus the neighboring region to be searched is determined by the number of grid points of the color-space correspondence table. The search time can be reduced by reducing the neighboring region. However, the neighboring region cannot be reduced alone. That is, in order to reduce the neighboring region, the number of grid points has to be increased. Thus, the reduction in the search time cannot be realized effectively.